This invention relates to a method of making films of piezoelectric materials suitable for preparing piezoelectric acoustic shear wave resonators and a method for making said resonators.
Recent developments in electronics technology have resulted in dramatic size reductions in electronic systems, particularly those utilizing active semi-conductor devices. While many passive components have kept pace with this size reduction, mechanical resonators have not. The need for miniaturized resonators is particulalry acute in the area of very large scale analog integrated circuits associated with communications and signal processing systems.
Attempts at manufacturing suitable high-frequency resonators have concentrated on the use of piezoelectric resonator plates. However, piezoelectric plates for very high and microwave frequencies are not amenable to the conventional processing technology associated with quartz or other traditional resonator materials. For example, plates less than 10 .mu.m thick are required in the VHF to microwave frequency range, but mechanical thinning of piezoelectric plates to the appropriate thickness is very difficult. In addition, for satisfactory resonance the plate surfaces must be parallel to a high precision.
It is known that resonators of suitable thinness and parallelism may be attained with piezoelectric films. These films may be manufactured by known means such as sputtering. Sputtered films of common piezoelectric materials show a strong tendency to grow with their C-axis perpendicular to the film substrate. These films are therefore suitable for longitudinal wave excitation when driven by an electric field perpendicular to the film plane.
For many applications it would be desirable to have a resonator suitable for excitation of shear waves rather than longitudinal waves. Shear wave excitation is possible when the C-axes of 6 mm piezoelectric crystallites lie in the substrate plane and the crystallites are aligned with one another. It is known that such films may be deposited on a substrate of zinc, In.sub.2 O.sub.3 or In.sub.2 O.sub.3 /SnO.sub.2, as described in U.S. Pat. No. 3,846,649 to Lehmann et al. disclosing a piezoelectric transducer. However, at the present time there is no satisfactory technique for growing C-axis in-plane piezoelectric films for general application.
It has been calculated that, for piezoelectric materials of 6 mm symmetry such as ZnO, AlN and CdS, a nearly pure shear wave would be excited for films having C-axis orientation substantially inclined at an acute angle with respect to the surface normal such that the shear wave coupling coefficient significantly exceeds the longitudinal wave coupling coefficient, as reported by N. F. Foster, et al., IEEE Trans. on Sonics and Ultrasonics, Vol. SU-15, No. 1, Jan., 1968, p. 28. Under these conditions, the quasi-shear-wave excitation will greatly exceed the quasi-longitudinal-wave excitation, and hence a good shear wave acoustic device can be made. A C-axis angle of inclination of about 40.degree. to about 50.degree. is preferred. Foster et al. acknowledged that at the time of their writing such films could not be made. Ibid. at 40. Attempts have since been made to sputter deposit C-axis inclined ZnO film by placing the substrate at a 90.degree. angle with respect to the target, as described by M. Minakata et al., Japan. J. Appl. Phys., Vol. 12, 1973, p. 474. However, the resulting films have uneven thickness and a mean C-axis variation unsuitable for device applications. In addition, it would be desirable for the response of the resonator to be relatively constant over a range of operating temperatures.